Percutaneous vertebroplasty is now commonly used to repair vertebrae which have become damaged or weakened, for example by osteoporosis, osteolytic spinal tumours, and the like. The gradual loss of bone minerals and progressive structural change of the trabecular bone which occur in osteoporosis result in vertebral fragility fractures. Vertebroplasty is used to improve the structural integrity of such mechanically weakened vertebrae affected by osteoporosis or tumors. This procedure involves the injection of viscous bone cement into the trabecular bone of the vertebral body. The bone cement, once hardened, becomes a permanent reinforcement of the vertebral body and usually drastically diminishes the pain experienced by the patient.
Transpedicular vertebroplasty is the most commonly used approach to access the vertebral body, however other approaches are also known, such as posterolateral and intertransverse. Transpedicular vertebroplasty involves the insertion of a cannula through the patient's skin, through the pedicle of the vertebra, and into the vertebral body. The vertebral body is then filled with bone cement, fed through the cannula, which solidifies within the vertebral body thereby stabilizing and strengthening the damaged vertebra.
Much of the equipment used to date for transpedicular vertebroplasty has been “off the shelf” surgical tools, which were originally designed for other procedures but which has been adapted for use with this procedure. As a result, the transpedicular vertebroplasty procedure itself has not to date been optimized, such as to improve the ease of performing this surgical procedure while reducing the risk to both patient and surgeon.
Further, improvement is sought for several different aspects of current equipment used for transpedicular vertebroplasty. For example, one risk inherent with the transpedicular vertebroplasty is the potential for bone cement leakage out of the vertebral body and into the venous system or into the spinal canal, which can cause serious, life threatening complications. Many of the more recent attempts to provide improvements have been focused on this point. The bone cement is believed to leak because it is injected at a low viscous or liquid like state. While increasing the viscosity of the bone cement injection has been associated with fewer leaks, thereby improving the safety of the procedure, a large injection force is required in order to be able to generate a pressure which is sufficiently high to displace the cement. To generate these pressures, some clinicians have resorted to using small volume syringes (ex: 1 cc to 3 cc) to inject bone cement, because the smaller cross sectional area of such small syringes permits generating higher pressure to displace the higher viscosity bone cement which can still be generated by the surgeon using a one-handed pincer grip. The inherent disadvantage of such smaller syringes is that many are required to inject the recommended amount of cement into a single vertebral body (typically 6-8 cc in the lumbar region, maximum 10 cc). Also, small syringes lack the volumetric stiffness and strength of components (e.g., the plunger) to handle sufficiently high pressures (3-5 MPa and higher). The use of several small syringes is therefore time consuming and less than ideal. Filling and using multiple syringes requires the clinician to repeatedly change syringes, which can distract attention away from the procedure at hand and any potentially dangerous complications which may occur, such as leakage of the cement for example. Still other disadvantages of working with multiple small syringes are that the procedure is time consuming, messy, and filling multiple small syringes ahead of time with cement may cause the syringe nozzle to clog.
Several different prior art methods and devices exist, all of which attempt to solve this problem (i.e. the generation of sufficient pressure to be able to inject bone cements having higher viscosities), however all have disadvantages. For example, some such devices are large and bulky, and employ large hand lever pumps or power screws to displace the cement. The significant weight and bulk of such devices makes them less practical and unsuitable for mounting directly atop a bone biopsy cannula, because the weight may bend the cannula and fracture the osteoporotic pedicle. As a result, these devices must be connected to the cannula via a long, small diameter tubing. Long tubing is also used to connect the injection device with the cannula to avoid radiation that the surgeon's hand may otherwise be exposed to when manipulating the device in the radiation field of a fluoroscope. Fluoroscopes are routinely used for cement injection into vertebra, with the intent being to immediately visualize adverse cement flow. Unfortunately, the friction of the cement flowing through such a long small diameter tube is extremely high and, as a result, almost all of the force generated by the gun, pump or power screw is used to overcome this friction within the piping. Further, these large systems dramatically limit the tactile feel of the surgeon and their sheer size become very cumbersome and expensive when three or four units are required for use simultaneously, such as during a multi-segmental procedure.
Another challenge facing surgeons performing vertebroplasty is the determination of when the bone cement is ready to be injected into the vertebral body. The surgeon must therefore decide when the cement has reached an acceptable level of polymerization to permit safe injection thereof. This is often done by simply extruding a small sample of the cement from the end of the injector being used, and the surgeon manually determines based on the tactile feel of the material whether it “feels doughy”. This method is clearly subjective, and further the sample taken may not be representative of the remaining cement in the injector, which may potentially be polymerizing at a different rate depending on a number of factors, including for example the thermal transfer properties of the reservoir material, room temperature and humidity, and the heat transferred from the hand of the surgeon to the sample as it is mechanically massaged.
Governmental organizations have also recently begun issuing notices to hospitals within their jurisdictions related to safety information on the use of bone cements, particularly for vertebroplasty and kyphoplasty operations. Accordingly, it is becoming increasingly important for surgeons to be able to accurately, effectively and safely inject bone cement, and thus continued improvement for devices employed to inject bone cement, and particularly high viscosity bone cement, is desired.
Some attempts at developing improved devices for the injection of high viscosity bone cement have been made. However, improvements continue to be sought, both in the design of the injector device itself as well as the entire system employed therewith for injecting high viscosity bone cement into patients, such as may be used, for example, when performing percutaneous vertebroplasty and/or other procedures used to improve the structural integrity of a given bone element.